Method for the preparation of Li1+αV3O8

ABSTRACT

The invention relates to a lithium vanadium oxide which corresponds to the formula Li 1+α V 3 O 8  (0.1≦α≦0.25). It is composed of agglomerates of small needles having a length l from 400 to 1000 nm, a width w such that 10&lt;l/w&lt;100 and a thickness t such that 10&lt;l/t&lt;100. It is obtained by a process consisting in preparing a precursor gel by bringing α-V 2 O 5  and a Li precursor into contact in amounts such that the ratio of the concentrations [V 2 O 5 ]/[Li] is between 1.15 and 1.5 and in subjecting the gel to a heat treatment comprising a first stage at 80° C.-150° C. for 3 h to 15 days and a second stage between 250° C. and 350° C. for 4 min to 1 hour, under a nitrogen or argon atmosphere. It is useful as an active material of a positive electrode.

The present invention relates to a process for the preparation of alithium vanadium oxide, to the compound obtained and to its use asactive material of a positive electrode.

BACKGROUND OF THE INVENTION

Batteries comprising a positive electrode and a negative electrodeseparated by an electrolyte comprising a lithium salt in solution in asolvent are widely known. The operation of these batteries is providedby the reversible circulation of lithium ions in the electrolyte betweenthe electrodes. The positive electrode is generally composed of acomposite material comprising an active material, a binder, a materialconferring electron conduction and optionally a compound conferringionic conduction. The compound conferring electron conduction can be acarbon black which does not catalyze the oxidation of the electrolyte ata high potential.

The use is known of lithium vanadium oxides Li_(1+α)V₃O₈ (0.1≦α≦0.25) aspositive electrode active material. Various processes for thepreparation of these compounds are known. A particularly advantageousprocess consists in preparing a precursor gel of the mixed oxide, indrying the gel and in then subjecting it to a heat treatment. Thus, S.Jouanneau et al. [J. Mater. Chem., 2003, 13, 921-927] describe a processwhich consists in preparing a gel by adding V₂O₅ to an aqueous LiOH.H₂Osolution, in drying the gel obtained after maturing at 50° C. withstirring for 24 hours and in then subjecting the xerogel obtained to aheat treatment at 350° C. or at 650° C. for 10 hours. The disadvantageof this process is that it employs a heat treatment over a long periodof time, the energy thus required significantly increasing theproduction cost.

The present inventors have now found that an oxide Li_(1+α)V₃O₈ havingproperties at least equivalent to those of the oxide of the above priorart can be obtained by a similar process in which the heat treatment iscarried out for a time ranging from a few minutes to 1 hour.

The aim of the present invention is to provide a simple and inexpensiveprocess for the preparation of an oxide Li_(1+α)V₃O₈ (0≦α≦0.25).

SUMMARY OF THE INVENTION

The process according to the present invention consists in preparing aprecursor gel and in subjecting said gel to a heat treatment. It ischaracterized in that:

-   -   the precursor gel is prepared by bringing α-V₂O₅ and a Li        precursor into contact in amounts such that the ratio of the        [V₂O₅]/[Li] concentrations is between 1.15 and 1.5;    -   the heat treatment is carried out in two stages: a first stage        at a temperature of between 80° C. and 150° C. for a time of 3        hours to 15 days and a second stage at a temperature of between        250° C. and 350° C. for a time of between 4 min and 1 hour,        under air or under a nitrogen or argon atmosphere.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 represents the X-ray diffraction diagram for the various samplesobtained and for the initial sample before heat treatment at 350° C.

FIG. 2 represents the variation in the capacity as a function of thenumber of cycles for the first series.

FIG. 3 represents the variation in the capacity as a function of thenumber of cycles for the second series.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In a first embodiment, the Li precursor is LiOH.H₂O. α-V₂O₅ and LiOH.H₂Oare introduced into the water under a nitrogen atmosphere and the gel isformed in approximately 15 h. The concentrations of precursors can varybetween 0.75 mol/l and 3 mol/l for α-V₂O₅ and between 0.55 mol/l and 2.2mol/l for LiOH.H₂O.

In a second embodiment, an aqueous solution comprising from 10 to 50% byvolume of hydrogen peroxide is added to the reaction medium. The gel isthen formed in a few minutes. The limiting concentrations which can beused are from 0.05 mol/l to 2 mol/l for α-V₂O₅ and from 0.04 mol/l to1.5 mol/l for the Li precursor.

In the second embodiment:

-   -   the lithium precursor can be chosen from LiOH.H₂O, LiCl, LiNO₃        or a lithium salt of a carboxylic acid, for example chosen from        lithium acetylacetonate, lithium acetate, lithium stearate,        lithium formate, lithium oxalate, lithium citrate, lithium        lactate, lithium tartrate or lithium pyruvate;    -   α-V₂O₅ is brought into contact with an aqueous peroxide solution        in the presence of a lithium precursor. The gel is observed to        start forming after a few minutes. The gel is completely formed        after maturing for 15 min.    -   the respective amounts of Li precursor and of α-V₂O₅ in the        reaction medium are preferably such that 0.08 mol·l⁻¹<[Li]<0.7        mol·l⁻¹; 0.1 mol·l⁻¹<[V₂O₅]<1 mol·l⁻¹. Excessively high        concentrations of reactants can bring about effervescence, while        excessively low concentrations give precipitates and not gels.

The material obtained by the process of the invention is an oxideLi_(1+α)V₃O₈ (0.1≦α≦0.25) composed of agglomerates of small needles,said needles having a length l from 400 to 1000 nm, a width w such that10<l/w<100 and a thickness t such that 10<l/t<100.

A material according to the present invention can be used for thepreparation of a composite positive electrode for a lithium battery.

In a particular embodiment, a positive electrode according to thepresent invention is composed of a composite material which comprises:

-   -   an oxide Li_(1+α)V₃O₈ obtained by the process of the present        invention,    -   a binder conferring mechanical strength,    -   a compound conferring electron conduction,    -   optionally a compound conferring ionic conduction.

The content of oxide Li_(1+α)V₃O₈ is preferably between 80 and 90% byweight. The content of binder is preferably less than 10% by weight. Thecontent of compound conferring electron conduction is preferably between5 and 15% by weight. The content of compound conferring ionic conductionis preferably less than 5% by weight.

The binder can be composed of a nonsolvating polymer, of a solvatingpolymer or of a mixture of solvating polymer and of nonsolvatingpolymer. It can additionally comprise one or more liquid polar aproticcompounds. The nonsolvating polymer can be chosen from vinylidenefluoride homopolymers and copolymers, copolymers of ethylene, ofpropylene and of diene, tetrafluoroethylene homopolymers and copolymers,N-vinylpyrrolidone homopolymers and copolymers, acrylonitrilehomopolymers and copolymers, and methacrylonitrile homopolymers andcopolymers. Poly(vinylidene fluoride) is particularly preferred. Thenonsolvating polymer can carry ionic functional groups. Mention may bemade, as examples of such a polymer, of polyperfluoroether sulfonatesalts, some of which are sold under the name Nafion®, and polystyrenesulfonate salts.

The solvating polymer can be chosen, for example, from polyethers oflinear, comb or block structure, which may or may not form a network,based on poly(ethylene oxide); copolymers comprising the ethylene oxideor propylene oxide or allyl glycidyl ether unit; polyphosphazenes;crosslinked networks based on polyethylene glycol crosslinked byisocyanates; copolymers of oxyethylene and of epichlorohydrin; andnetworks obtained by polycondensation and carrying groups which makepossible the incorporation of crosslinkable groups.

The polar aprotic compound can be chosen from linear or cycliccarbonates, linear or cyclic ethers, linear or cyclic esters, linear orcyclic sulfones, sulfamides and nitriles.

The compound conferring electron conduction can be chosen, for example,from carbon blacks, graphites, carbon fibers, carbon nanowires or carbonnanotubes.

The compound conferring ionic conduction is a lithium saltadvantageously chosen from LiClO₄, LiPF₆, LiAsF₆, LiBF₄, LiR_(F)SO₃,LiCH₃SO₃, lithium bisperfluoroalkylsulfonimides or lithium bis- ortrisperfluorosulfonylmethides.

A composite positive electrode according to the invention can beprepared by mixing the oxide Li_(1+α)V₃O₈, a binder in an appropriatesolvent, a material conferring electron conduction, and optionally alithium salt, by spreading the mixture obtained over a metal disk actingas collector (for example an aluminum disk) and by then evaporating thesolvent under hot conditions under a nitrogen atmosphere. The solvent ischosen according to the binder used. In addition, a positive electrodecan be prepared by extrusion of a mixture of its constituents.

An electrode thus constituted can be used in a battery comprising apositive electrode and a negative electrode separated by an electrolytecomprising a lithium salt in solution in a solvent. The operation ofsuch a battery is provided by the reversible circulation of lithium ionsin the electrolyte between the electrodes. One of the subject matters ofthe present invention is a battery in which the electrolyte comprises alithium salt in solution in a solvent, characterized in that itcomprises a positive electrode comprising, as active material, the oxideLi_(1+α)V₃O₈ prepared according to the process of the present invention.When a positive electrode comprising the oxide Li_(1+α)V₃O₈ as obtainedby the process of the invention is fitted into a battery, the batterythus formed is found in the charged state.

In a battery according to the invention, the electrolyte comprises atleast one lithium salt in solution in a solvent. Mention may be made, asexamples of salts, of LiClO₄, LiAsF₆, LiPF₆, LiBF₄, LiR_(F)SO₃,LiCH₃SO₃, LiN(R_(F)SO₂)₂, LiC(R_(F)SO₂)₃ and LiCF(R_(F)SO₂)₂, R_(F)representing a perfluoroalkyl group having from 1 to 8 carbon atoms or afluorine atom.

The solvent of the electrolyte can be composed of one or more polaraprotic compounds chosen from linear or cyclic carbonates, linear orcyclic ethers, linear or cyclic esters, linear or cyclic sulfones,sulfamides and nitriles. The solvent is preferably composed of at leasttwo carbonates chosen from ethylene carbonate, propylene carbonate,dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate. Abattery having a polar aprotic solvent electrolyte generally operates ina temperature range from −20° C. to 60° C.

The solvent of the electrolyte can additionally be a solvating polymer.Mention may be made, as examples of solvating polymers, of polyethers oflinear, comb or block structure, which may or may not form a network,based on poly(ethylene oxide); copolymers comprising the ethylene oxideor propylene oxide or allyl glycidyl ether unit; polyphosphazenes;crosslinked networks based on polyethylene glycol crosslinked byisocyanates; copolymers of oxyethylene and of epichlorohydrin asdisclosed in FR-2 770 034; and networks obtained by polycondensation andcarrying groups which make possible the incorporation of crosslinkablegroups. Mention may also be made of block copolymers in which someblocks carry functional groups which have redox properties. A batteryhaving a polymeric solvent electrolyte generally operates in atemperature range from 60° C. to 120° C.

In addition, the solvent of the electrolyte can be a mixture of a liquidpolar aprotic compound chosen from the polar aprotic compounds mentionedabove and of a solvating polymer. It can comprise from 2 to 98% byvolume of liquid solvent, depending on whether a plasticized electrolytewith a low content of polar aprotic compound or a gelled electrolytewith a high content of polar aprotic compound is desired. When thepolymeric solvent of the electrolyte carries ionic functional groups,the lithium salt is optional.

The solvent of the electrolyte can also be a mixture of a polar aproticcompound as defined above or of a solvating polymer as defined above andof a nonsolvating polar polymer comprising units comprising at least oneheteroatom chosen from sulfur, oxygen, nitrogen and fluorine. Such anonsolvating polymer can be chosen from acrylonitrile homopolymers andcopolymers, fluorovinylidene homopolymers and copolymers, andN-vinylpyrrolidone homopolymers and copolymers. In addition, thenonsolvating polymer can be a polymer carrying ionic substituents and inparticular a polyperfluoroether sulfonate salt (such as anabovementioned Nafion®, for example) or a polystyrene sulfonate salt.

In another embodiment, the electrolyte of the battery of the presentinvention can be an inorganic conducting solid chosen from the compoundsgenerally denoted by Lisicon, that is to say Li₄XO₄—Li₃YO₄ (X=Si or Geor Ti; Y=P or As or V), Li₄XO₄—Li₂AO₄ (X=Si or Ge or Ti; A=Mo or S),Li₄XO₄—LiZO₂ (X=Si or Ge or Ti; Z=Al or Ga or Cr), Li₄XO₄—Li₂BXO₄ (X=Sior Ge or Ti; B=Ca or Zn), LiO₂—GeO₂—P₂O₅, LiO₂—SiO₂—P₂O₅,LiO₂—B₂O₃—Li₂SO₄, LiF—Li₂S—P₂S₅, Li₂O—GeO₂—V₂O₅ or LiO₂—P₂O₅—PON solidsolutions. A lithium battery comprising such an electrolyte operateswithin a very broad temperature range, of the order of −20° C. to 100°C.

Of course, the electrolyte of a battery of the present invention canadditionally comprise the additives conventionally used in this type ofmaterial and in particular a plasticizer, a filler, other salts, and thelike.

The negative electrode of the battery can be composed of lithium metalor a lithium alloy which can be chosen from the alloys β-LiAl, γ-LiAl,Li—Pb (for example Li₇Pb₂), Li—Cd—Pb, Li—Sn, Li—Sn—Cd, Li—Sn in variousmatrices, in particular oxygen-comprising matrices or metal matrices(for example Cu, Ni, Fe or Fe—C), or Li—Al—Mn.

In addition, the negative electrode of the battery can be composed of acomposite material comprising a binder and a material capable ofreversibly inserting lithium ions at low redox potential (hereinafterdenoted by insertion material), said composite material being lithiatedduring a preliminary stage. The insertion material can be chosen fromnatural or synthetic carbonaceous materials. These carbonaceousmaterials can, for example, be a petroleum coke, a graphite, a graphitewhisker, a carbon fiber, mesocarbon microbeads, a pitch coke or a needlecoke. The insertion material can additionally be chosen from oxides,such as, for example, Li_(x)MoO₂, Li_(x)WO₂, Li_(x)Fe₂O₃, Li₄Ti₅O₁₂ orLi_(x)TiO₂, or from sulfides, such as, for example, Li₉Mo₆S₆ and LiTiS₂,or from oxysulfides. Use may also be made of compounds which make itpossible to reversibly store lithium at low potential, such as amorphousvanadates (for example Li_(x)NiVO₄), nitrides (for exampleLi_(2.6−x)Co_(0.4)N, Li_(2+x)FeN₂ or Li_(7+x)MnN₄), phosphides (forexample Li_(9−x)VP₄), arsenides (for example Li_(9−x)VAs₄) andreversibly decomposable oxides (for example CoO, CuO or Cu₂O). Thebinder is an organic binder which is electrochemically stable in therange of operation of the negative electrode. Mention may be made, byway of examples, of poly(vinylidene fluoride) homopolymers or anethylene/propylene/diene copolymer. A poly(polyvinylidene fluoride) isparticularly preferred. A composite negative electrode can be preparedby introducing the carbonaceous compound into a solution of the binderin a polar aprotic solvent, by spreading the mixture obtained over acopper disk acting as collector and by then evaporating the solventunder hot conditions under a nitrogen atmosphere.

A battery according to the invention comprising a solid electrolyte canbe provided in the form of a succession of layers composed respectivelyof the material of the positive electrode according to the invention andits current collector, the solid electrolyte, and the negative electrodeand optionally its current collector.

A battery according to the invention comprising a liquid electrolyte canalso be provided in the form of a succession of layers composedrespectively of the material of the positive electrode according to theinvention and its current collector, a separator impregnated by theliquid electrolyte, and the material constituting the negative electrodeand optionally its current collector.

The present invention is illustrated in more detail by the examplesgiven below, to which, however, it is not limited.

EXAMPLE 1

An aqueous solution of precursors was prepared by adding 6.8200 g (1.5M)of α-V₂O₅ and 1.2589 g (1.2M) of LiOH.H₂O to 25 ml of water under anitrogen atmosphere. A gel G was formed after a maturing period of 15hours. Subsequently, the gel was dried at 90° C. in the air for 15 daysand then the xerogel thus obtained was subjected to a treatment at 350°C. under argon for 4 min. The product obtained is hereinafter denoted byXG-4.

EXAMPLE 2

Other samples were prepared according to the procedure of example 1while modifying the drying time (DT) and/or the heat treatment time(HTT), as shown in table 1 below, in which the sample XG-4 from example1 is mentioned for the record.

Sample XG-4 XG-2 XG-5 XG-15a XG-15b XG-15c XG-60 DT 15 d 15 d 15 d 15 d3 d 12 d 15 d HTT 4 2 5 15 15 15 60 (min)

EXAMPLE 3

1 g of V₂O₅ and 0.1689 g of LiOH were added to 15 ml of a 30% aqueoushydrogen peroxide solution. A gel was formed in a few minutes.

The gel obtained was subjected to drying at 90° C. in the air overnightand then to a heat treatment at 350° C. under argon for 15 min.

EXAMPLE 4

Other samples were prepared by repeating the procedure of example 1 butcarrying out the first stage of the heat treatment for a time of 12hours at 90° C. in the air and for different treatment times (HTT) at350° C. as follow, in minutes: 1, 2, 3, 4, 5, 15, 30 and 45.

FIG. 1 represents the X-ray diffraction diagram for the various samplesobtained and for the initial sample before heat treatment at 350° C. Itis thus confirmed that a heat treatment of 4 min at 350° C. issufficient to clearly reveal the compound Li_(1+α)V₃O₈ of the presentinvention, the line of which is labelled x in the diagram.

EXAMPLE 5 Measurement of the Performances

The electrochemical performances of various samples of oxide were testedin a Swagelok laboratory battery of the type: Li/liquid electrolyte(EC+DMC+LiPF₆)/(XG+carbon), operating at ambient temperature. For thepositive electrode, carbon black was added to the sample of oxide XG.

A first series of measurements was carried out under cycling conditionscorresponding to 2.5 Li per formula group pert hour, on the one handwith the samples XG-2, XG-4, XG-15a, XG-15b and XG-60 according to theinvention and, on the other hand, with a sample XG-0. This sample XG-0was obtained according to the process of the example, without the heattreatment at 350° C.

A second series of measurements was carried out under cycling conditionscorresponding to 0.4 Li per formula group per hour, on the one hand withthe sample XG-15c according to the invention and, on the other hand,with a sample SG350. This sample SG350 was obtained under the conditionsof the sample XG-15 but with a heat treatment at 350° C. for 10 h,preceded by a rise in temperature at the rate of 80° C./h.

FIGS. 2 and 3 represent the variation in the capacity as a function ofthe number of cycles for the first series and for the second seriesrespectively.

It is thus apparent, from FIG. 2, that, for a cycling rate of 2.5 Li perhour per formula unit, the heat treatment time at 350° C. can be reduceddown to 4 min without obtaining a significant loss in capacity duringsuccessive cycles.

FIG. 3 shows that, for a cycling rate of 0.4 Li per hour per formulaunit, the heat treatment can be reduced from approximately ten hours to15 min without significant loss in initial capacity and with a markedlyimproved retention in capacity during the cycles.

What is claimed is:
 1. A process for the preparation of an oxideLi_(1+α)V₃O₈ (0≦α≦0.25) comprising preparing a precursor gel andsubjecting said gel to a heat treatment, wherein: the precursor gel isprepared by bringing α-V₂O₅ and a Li precursor into contact in areaction medium in amounts such that the ratio of the [V₂O₅]/[Li]concentrations is between 1.15 and 1.5; and the heat treatment iscarried out in two stages: a first stage under air at a temperature ofbetween 80° C. and 150° C. for a time of 3 hours to 15 days and a secondstage under a nitrogen or argon atmosphere at a temperature of between250° C. and 350° C. for a time of between 4 min and 1 hour.
 2. Theprocess as claimed in claim 1, wherein the Li precursor is LiOH.H₂O andα-V₂O₅ and LiOH.H₂O are introduced in water under a nitrogen atmosphere.3. The process as claimed in claim 2, wherein the concentration ofα-V₂O₅ is between 0.75 mol/l and 3 mol/l and the concentration ofLiOH.H₂O is between 0.55 mol/l and 2.2 mol/l.
 4. The process as claimedin claim 1, further comprising adding an aqueous solution comprisingfrom 10 to 50% by volume of hydrogen peroxide to the reaction medium. 5.The process as claimed in claim 4, wherein the concentrations are from0.05 mol/l to 2 mol/l for α-V₂O₅ and from 0.04 mol/l to 1.5 mol/l forthe Li precursor.
 6. The process as claimed in claim 4, wherein thelithium precursor is LiOH.H₂O, LiCl, LiNO₃ or a lithium salt of acarboxylic acid.
 7. The process as claimed in claim 6, wherein thelithium salt of carboxylic acid is lithium acetylacetonate, lithiumacetate, lithium stearate, lithium formate, lithium oxalate, lithiumcitrate, lithium lactate, lithium tartrate or lithium pyruvate.
 8. Theprocess as claimed in claim 4, wherein α-V₂O₅ is brought into contactwith an aqueous peroxide solution in the presence of a lithiumprecursor.
 9. The process as claimed in claim 4, wherein the respectiveamounts of Li precursor and of α-V₂O₅ in the reaction medium are suchthat 0.08 mol/l<[Li]<0.7 mol/l and 0.1 mol/l<[V₂O₅]<1 mol/l.